1. Field of the Invention
The present invention relates to a nuclear reactor of the type in which the cooling water has a free liquid level in the pressure vessel such as in the boiling-water reactor, so that even when a break has developed in the drain conduit in a system which circulates the cooling water into the pressure vessel in the reactor, the cooling water is prevented from escaping, and the cooling water is allowed to remain in sufficiently large amounts in the pressure vessel.
2. Description of the Prior Art
A conventional boiling-water reactor is equipped, with a cooling water recirculation system which consists of recirculation conduits recirculation pumps, and jet pumps, in order to supply large amounts of cooling water to a reactor core to enhance the core efficiency. From the necessity to take out the cooling water of a relatively low temperature from the pressure vessel of the reactor, an intake of the drain conduit in the cooling water recirculation system is opened in the pressure vessel at a position below the reactor core. In such a nuclear reactor, if breakage develops in the drain conduit, the cooling water gushes out through a broken portion since the pressure is high in the pressure vessel, the water level in the pressure vessel drops quickly to the position of intake of the conduit, and the reactor core burns without the cooling water.
Another nuclear reactor in which the cooling water has a free liquid level in the pressure vessel of nuclear reactor can be represented by a nuclear reactor of the type of natural circulation. In the nuclear reactor of this type, provision is usually made of a purifying system or a spray system for pressure control between the interior and the exterior of the pressure vessel to circulate the cooling water while purifying the cooling water or controlling the pressure in the pressure chamber. In the former system, the intake of the drain conduit is opened in the lower portion of the pressure vessel so that precipitates deposited on the lower portion of the pressure vessel can be removed. In the latter system, also, the intake of the drain conduit is opened in the lower portion of the pressure vessel so that cold cooling water can be taken out. Even in such a nuclear reactor, if the drain conduit breaks, the water level in the pressure vessel may drop to the level of intake of the drain conduit, and the reactor core may burn without the cooling water.